Fin question

[johnsonm50]

Fin question, Ive got 2 older restored hydros I run occasionally at teeker meets & CSH. The latest restore I noticed turns tighter & more predictably. Noting the newer fin is different I get the idea to copy it & try on the older boat. Of course its never that easy.. On test running it will wiggle before setting at speed. Once set its good, just looking for why that might happen. I checked both with a straight edge to the boat & their good. On the test run I did have it deeper than the old skinny dagger fin so I already set it shallower for the next test. It was also fully forward so could angled back a tad be my answer? If or when I find out Ill post it, any thoughts welcome. Thanks.

[ZUL8TR]

fin

are the mounting brackets the same on both hydros?
is there more flex of the fin on the hydro with the problem?
is the fin set vertical on both?
do both hydros have the same afterplane length? what are they?
do both hydros have full air traps?
do both traps end at the bottom or below the bottom?
a picture would help
maybe it is not the fin causing the problem.

[johnsonm50]

are the mounting brackets the same on both hydros?
is there more flex of the fin on the hydro with the problem?
is the fin set vertical on both?
do both hydros have the same afterplane length? what are they?
do both hydros have full air traps?
do both traps end at the bottom or below the bottom?
a picture would help
maybe it is not the fin causing the problem.

The mount brackets are different but essentially the same.
The copy I made is stainless steel & has less flex than the one that's working well. They are both about 1/16 in to the boat from vertical my thinking is that they might be vertical while under stress.
The Afterplanes are different, the one that turns good has 71" & full traps about 1/4 deep aft.
The one I'm working with has 70" & the traps end at 64 [they don't disappear to '0' but are about 1/4 deep ending square] The boat I'm working with is the faster of the 2 so far [another issue] & handled good I thought till I tested the other. Before changing the fin it had a curved dagger type fin I bent it once in turn 4, it would flex alot. The new one is a copy of a BTM in SS I'm thinking it was too deep, I raised it 3/4" & might have been angled forward a little I re-cut the curve & set it back a little. I'm going to run it at a 1/4 mi. run today & if possible Ill throw it into a few turns before or after to see if the changes- change. Ill get pics later & post how the 1/4 went. Thanks Pete, Mike

[trident]

Fin Material

Please don't use stainless steel for fins on the faster, more powerful boats.

My hard learned experience is stainless is great, to a point, then it yields suddenly, and you have no fin, in effect. I bent one totally horizontal on an EMH in the first turn long ago. Luckily, no crash into anyone, but my race was over. Could have been a really bad thing.

I'd recommend 1/8" 4130 steel or 3/16" 6061T6 aluminum.

Other key is to support the fin well with the bracket... a long unsupported length adds flex. I also put a rocker or radius at the lower edge where the fin attaches to the bracket, to not create a sharp shear edge...

Hope this helps.

Jerry

[trident]

Turn Fins

To quote Yogi, 'its like deja vu all over again'

I knew we had been down this road before.

Much earlier thread on this... Turning Fin Info, 9-2009

Try searching for it... might be worth a read, reread.

Jerry

[sam]

I had a Pugh that I "straightened" the fin bracket ... it gave it a bad wiggle on setting ... I returned the bracket and fin angle to slightly canted in at the bottom the way Pugh put it on the boat. Wiggle went away. A case of builder knows best and straight up and down may not be ideal for every boat.

[bill van steenwyk]

Never tried to figure out the physics behind the "canted" fin installation, other than the obvious thoughts about it, but all the Hydros I ever had, had a canted fin, either from the builder, and some I built myself. They all turned very well with no hooking or other bad habits, and would hold their line in a corner. Also no "wiggle" or whatever has been described by other posters.

I saw numerous canted fin installations on Unlimited boats (some that were very extreme) and other limited inboards in the 70's and 80's. Whether that is where the idea came from I don't know.

When you think about it, it would seem to make sense, and also as related, in my experience anyway, nothing bad came from it. If you are always turning left, and the fin is canted from left to right, top to bottom, (when viewed from the rear) because then when the boat "leans over" to the right in the turn while trying to lower the outboard sponson, you should end up with a fin that is more or less straight up and down, or at least not as "off center" as it would be if not canted. That also would be the reason for offset cockpits to the left, in always left turning boats. There is another school of thought that reasons that if the chine angles are correct, it will accomplish this same thing.

Sometimes if something works, which this always seemed to for me, just use it and let someone else try to figure it out.

I always got a kick out of folks (usually front runners) that would put something on their boat that was different, whether it helped or not, and then wait to see how many copied it. It usually didn't take long for others to have the same thing on their boats.

[johnsonm50]

I took this hydro to run 1/4 mile speed runs today & the changes I made improved it some as much as I could figure out from turning off the course on the gas & a few milling circles. The wiggle is gone so Im thinking it was too deep.
This is a C so I hope its ok [not to bend or break] but might just get an aluminum one from BTM. I remember several fin sessions too but don't remember the 'wiggle' being ironed out, kind funny & the same time you might think 'this could be a problem'. Thanks for the replies.

[bill van steenwyk]

Sometimes if your steering cables are a little loose, or possible the swivel pin has some play in it, or anything that will allow the motor to "vibrate" back and forth, JUST THE SLIGHTEST, can cause a problem like you describe. If you are using tension springs where the cables attach to the steering bar, you might try taking them out and make sure the cables are tight.

[johnsonm50]

Thanks Bill its regular bar & tie back steering no springs.. but were the cables tight enough [?] is another possibility. After running it today the wiggles gone & its about the same as before.